This invention relates to an extrusion die assembly for forming honeycomb structures from extrudable materials such as glass, glass-ceramics, ceramics, plastics, metals, cermets and other materials, especially those in particulate form, which are capable of being extruded through relatively small feed holes or channels whose length is several times their diameter or transverse dimension. The outlet ends of the feed holes communicate with grid forming discharge slots, these slots forming the cell walls of a honeycomb form extrudate. After the extrusion process, the honeycomb is treated to produce a rigid honeycomb structure as is known in this art.
Thin-walled honeycomb structures display utility in a variety of technologies. For example, thin-walled honeycomb structures fashioned from ceramic materials are used as catalyst carriers in catalytic converters in the exhaust system of internal combustion engines. They also are employed as radiators, catalyst carriers, filters, diesel particulate filters, molten metal filters, woodstove combustor substrates, and heat exchangers.
This invention more specifically relates to an extrusion die assembly for producing a thickened skin or thickened outer wall on the surface of the extruded honeycomb structure. Without such a thickened skin, the external surface of the extrudate is defined by elongated ridges, the individual ridges being the exposed outer walls of the outermost honeycomb passageways of the fasces-like extruded structure or extrudate. The thickness of the outer surface is merely the wall thickness of the individual honeycomb passageways which form the elongated ridges. Workers in this art have recognized that the mechanical strength of the extrudate can be increased by modifying the extrusion process so that the extruded structure will have a thickened outer surface or skin so as to be thicker than the honeycomb cell walls. This is usually effected by deforming or crushing the elongated honeycomb cells at the periphery of the extrudate as the extrudate emerges from the die. This has been done by providing a ring with a radially inwardly extending lip, the lip distorting or changing the axial output direction of the flowable material passing through the periphery of the die discharge slots. The ring is often spaced from the face of the discharge slots by a shim, the region between the lip, shim and discharge slots forming a shim reservoir for the batch material being extruded. Such a ring is often termed a mask. A backer plate and flow diverter have also been used, the latter two elements being positioned on the inlet or upstream side of the die. The mask, the die, the shim and other elements being referred to herein as an extrusion die assembly.
The prior art is aware of extrusion die and mask arrangements for forming thin-walled honeycomb structures having a thickened outer wall, such as shown in U.S. Pat. Nos. 4,278,412 issued to Ozaki, 4,368,025 issued to Niato, and 4,384,841 issued to Yamamoto.
While generally satisfactory for forming a thickened outer skin, these typical known arrangements cannot provide consistent batch flow to the shim reservoir to form thicker skin. Forming thicker skin results in higher levels of air checks, distortion, and rippled skin. Further, the use of a flow diverter/backer plate for flow control results in more misshape and fissures on high footage dies.